Objection about buying an app

I bought an app few month ago , and paid about 6 Euros with my son,s credit card.Recently he told me that from app store 600 Euros has been taken from his credit card for that app ( pocket earth pro) for what? we don,t know .I sent an email and they told me I should ask Apple .I would be greatful if you kindly help me to refund his money or know why it happend.

Shokouh